The present invention is generally related to the field of applications that employ terahertz radiation and, more particularly, at least to the detection of subcentimeter radiation in the frequency domain. Still more particularly, a system is described for investigation of the properties of a sample of matter using subcentimeter radiation in the frequency domain.
While the prior art includes a number of approaches for the production of terahertz energy, there are considered to be a limited number of approaches with respect to detection and analysis of such radiation. One prior art method for the detection of submillimeter radiation utilizes a Helium Bolometer. Such a device has a very slow response and requires liquid Helium to function. These characteristics make it unsuitable for a large number of applications. Another prior art method uses a high frequency mixer diode in conjunction with a multiplied sub-millimeter source to perform downconversion of the subject radiation to a lower frequency range. This latter technique is limited, however, since such mixer diodes are typically provided in waveguide structures that are characterized by a significantly limited bandwidth of operation. Still another detection method performs detection of sub-millimeter radiation in a semiconductor material using time domain analysis of sub-millimeter pulses.
It is noted that the terms THz and sub-millimeter are often used in the prior art to refer to at least a segment of the energy that is of interest herein. The term sub-millimeter (submm) may be considered as the more technically accurate of the two terms since it can be considered to refer to the wavelength in a vacuum as being less than or equal to 1 millimeter. While no precise definition is available, in the literature it appears that when the term “THz” is used, it refers generally to radiation in the frequency range of 0.1 THz to 10 THz. Since 1 mm is about 300 GHz, the “THz region” of the current technological parlance is really more than sub-mm. Based on these considerations, Applicants have adopted the term sub-centimeter (sub-cm) (˜>30 GHz) wherein the wavelength in a vacuum is less than or equal to approximately 1 centimeter.
One approach, in a paper by Smith, Auston and Nuss, describes the use of a photoconducting dipole antenna for purposes of generating and sensing terahertz radiation. As described, a balanced colliding pulse mode-locked laser, driven by an argon laser, is used for purposes of generating short duration optical pulses. The pulsed output is split into two beams of equal intensity. One beam is delivered to a transmitting antenna so as to excite the transmitting antenna to emit a burst of terahertz energy. A receiving antenna is spaced away from the transmitting antenna for receiving the burst of terahertz energy. The other beam of pulsed laser energy is first delayed such that it arrives at the receiving antenna at the same time as the burst of terahertz energy. The delayed laser pulsed energy thereby serves to gate the receiving antenna at the time of arrival of the terahertz pulse. Unfortunately, the gate pulse, at the receiving antenna, will have a duration that is far shorter than the received terahertz energy, since the latter will be subject to dispersion in traveling from the transmitter antenna to the receiver antenna. Thus, a very small sample of the dispersed terahertz energy will be received. Although one of these gated samples of the dispersed terahertz energy is relatively useless standing on its own, since it merely represents a very limited portion of an overall domain response, Smith et al. recognize a technique for obtaining useful information from a plurality of such pulses that are delayed by different amounts so as to reconstruct an overall time domain or temporal response. It is noted that this sort of analysis is nontrivial. Further, substantial complexity is introduced with respect to appropriately timing the gating pulse.
Accordingly, the approach of Smith et al. allows one, after collecting a significant number of time-gated samples, to perform a Fourier transform of the overall temporal response whereby to obtain a frequency response. Since achieving the resolution required for molecular spectroscopy would require a delay line of over a thousand meters, such a time domain based response is considered to be unsuitable. Further, the burst of terahertz energy emitted by the transmitting antenna is spread across a relatively wide bandwidth whereas, in a single, narrow bandwidth component, the radiated subcentimeter power would be greater, leading to improved signal-to-noise ratios for spectroscopic applications.
A logical extension of the work of Smith et al., therefore, is the application of what is referred to in the prior art as “terahertz time domain spectroscopy” wherein a sample to be investigated is placed between the source of terahertz energy and the detector. U.S. Pat. No. 5,623,145, issued to Nuss, represents the extension of this work by one of the co-authors of the Smith et al. article. In particular, the Nuss patent reportedly teaches time domain spectroscopy for the purpose of performing imaging. Applicants see no substantive change with respect to the overall gating technique.
The prior art has continued to develop with respect to terahertz time domain spectroscopy as demonstrated, for example, by U.S. Pat. No. 6,865,014. It is submitted, however, that no fundamental change in the basic operation of these systems has occurred in which the detector is gated by a delayed pulse. In particular, it is submitted that there is at least a need for a new scheme for detection of terahertz energy which avoids the difficulties encountered in the approach of time domain analysis. Moreover, frequency response is the objective of spectroscopy, as opposed to first obtaining a temporal response. Accordingly, time domain spectroscopy is submitted to be an indirect, limited and circuitous approach to spectroscopy, when frequency domain response is the objective in the first instance.
The present invention is submitted to sweep aside the foregoing difficulties and concerns while providing still further advantages.